1. Field of the Invention
The present invention relates generally to chelators and more specifically to fluorescent chelators.
2. Background of the Invention
Description of the Background Art
The use of fluorescence to detect metals provides a convenient way to measure low concentrations of metals. Generally, a sample is exposed to a fluorescent chelating agent that preferentially complexes with the metal of interest, either shifting or greatly enhancing the fluorescence of the complexed ligand. However, these fluorescent chelates may be subject to interference resulting in unreliable results and rarely exhibit the ability to preferentially distinguish between various related metals.
The dual fluorescence of 4-(N,N-dimethylamino) benzonitrile, DMABN (FIG. 1a), in polar solvent was discovered by Lippert more than 30 years ago. Lippert et al, Angew. Chem., 1961, 73, 695. Several models have been proposed to address the cause for the unexpected, red-shifted fluorescence band which is apparent in addition to the normal, fluorescent emission of the locally excited state (LE). Grabowski et al proposed the twisted intramolecular charge transfer (TICT) state to successfully account for the red-shifted fluorescence. Grabowski et al, Pure Appl. Chem., 1983, 55, 245. TICT refers to the state wherein an intramolecular charge transfer takes place between the dimethyl amino donor group and the benzonitrile acceptor that is accompanied by a twisting motion and orbital decoupling of the phenyl acceptor ring from the dimethyl amino donor group. The TICT state has been observed in other molecular systems bearing donor and acceptor groups covalently linked. When the N,N-dimethylamino group in DMABN is replaced by an amino propyl group, the resulting compound, 3-(4-cyanophenyl)-1-N,N-dimethylamino-propane, forms an intramolecular exciplex between the phenyl and amino end groups. The formation of an intramolecular exciplex results from the flexibility of the alkyl chain, which links the donor and acceptor moieties close enough spatially, that a charge transfer step is possible.
Research has been conducted toward the development of fluorescent sensor molecules capable of sensitively and selectively monitoring heavy metal ion concentration. The covalent attachment of benzonitrile to the aza group of a metal binding ionophore that uniquely combines the properties of DMABN with the metal binding properties of metal binding ionophore has been considered as one way to accomplish this goal. For example, Letard et. al., Recl. Trav. Chim. Pays-Bas, 1995, 114, 517-527, reported the dual fluorescence of 4-(1-aza-4,7,10-trioxacyclododecyl)benzonitrile (DMABN-Crown4, FIG. 1c) and 4-(1-aza-4, 7,10,13-tetraoxacyclopentadecyl)benzonitrile (DMABN-Crown5).
Benzonitriles bearing flexible, alkyl amino chains, e.g. 3-(4-cyanophenyl)-1-N,N-dimethylaminopropane (CNP3NM, FIG. 1b), can form intramolecular exciplexes which arise due to the conformational flexibility of the alkyl amino chain, and its ability to form a sandwich configuration promoting excited state, charge transfer (Grabowski, Z. R. Pure & Appl. Chem., 1992, 64, 1249-1255). The propensity for intramolecular exciplex formation in CNP3NM is indicated by the strong, red-shifted fluorescence observed, and the complete absence of LE emission. In contrast to TICT state formation, the large distance and minimal mesomeric interaction apparent between D and A causes a localized excited (LE) state precursor to be formed, wherein the excited state is primarily localized on either D or A, depending upon which gives the lower S.sub.1 energy. Van der Auweraer et. al., J. Phys. Chem., 1991, 95, 2083-2092, observed a correlation between intramolecular exciplex formation and Hirayama's rule (Hirayama, F. J. Chem. Phys., 1965, 42, 3163-3171), which states that the most stable sandwich conformation will arise when n=3 for a phenyl-(CH.sub.2).sub.n -NMe.sub.2 system.